There are many of optical oxygen sensing systems based on Ru, Pt, or Ir heavy metal luminophores. However, oxygen sensors based on non-heavy metal luminophores with unusual dual emissive properties (e.g., based on boron), particularly single component systems with biocompatible “green” polymers are very rare. Fluorescent boron difluoride dyes such as “bodipy” and boron diketonates possess large molar extinction coefficients and two-photon absorption cross sections, high emission quantum yields and sensitivity to the surrounding medium. These features have been exploited in lasers, imaging agents, molecular probes, and photosensitizers. As two-photon absorbers, they are compatible with optical imaging technologies employing tunable Ti:sapphire lasers (700-1100 nm). Focused, longer wavelength IR excitation corresponds with greater tissue penetration, and reduced cell damage and interference from biological absorbers. Boron difluoride diketonate dyes possess large dipole moments and their emission wavelength sensitive to the polarity of the surrounding medium. Thus, solvatochromic boron complexes serve as probes of their local environments.
A typical dual emissive system works as described below. Though boron dye fluorescence is well known, phosphorescence is usually only observed in the presence of toxic heavy atom substituents or additives (e.g., Pb, Tl, or halogens such as I, Br), at low temperatures, or in rigid, solid matrices, which can be difficult to process and often are not biocompatible and biodegradable. Phosphorescence is quenched by oxygen, which at room temperature more accurately and conveniently may serve as the basis for quantitative optical oxygen sensing. Single component, readily processable systems exhibiting both fluorescence and phosphorescence are rare and may be adapted for imaging and ratiometric sensing. Fluorescence (short emission lifetimes) serves as an invariant feature providing information to quantify and locate the dye/emitter, whereas phosphorescence (long emission lifetimes) is quenched to variable extents depending upon the amount of oxygen that is present. Phosphorescent materials with long emission lifetimes are more sensitive to oxygen, and may serve as highly sensitive oxygen sensors in low oxygen environments (food packaging, hypoxic tumor or cardiovascular tissues, tissue engineering matrices, etc.) Luminescent materials can also be used as photosensitizers, transferring energy to other molecules, and generating reactive species by light activation. For example, this feature is exploited in photodynamic therapy, generating reactive singlet oxygen to selectively damage tumor tissue, and in lithography with two-photon dyes.
Currently, there is a need for compounds that have the ability to both fluoresce and phosphoresce. There is also a need for sensors that can be used to detect oxygen at low levels in tumors and cardiovascular tissue.